Magnetooptical disk memories are known to be memories in which the data are carried by magnetic disks and ready by opto-electronic devices. These disk memories are highly important solutions, since they enable attaining radial and longitudinal densities on the order of 10,000 tracks per centimeter and 10,000 data per centimeter, respectively. (Radial density is the number of data per unit of length measured along the diameter of the disk, while longitudinal density is the number of data per unit of length measured along the circumference of a track.)
In magnetooptical memories, one often-used mode of writing data is the "thermo-magnetic" mode, which comprises combining the thermal action due to the impact of a laser beam on the recording surface of the magnetic disk, and the application of a magnetic field to the vicinity of the impact. A discussion of the "thermo-magnetic" mode is found in U.S. Pat. No. 4,510,544. In the vicinity of the laser beam impact, the heating of the magnetic medium is on the order of several tens of degrees and may even exceed a hundred degrees.
The mode of operation of magnetooptical disk memories is based on the magnetooptic effect, which is characteristic of certain magnetic materials, in particular the alloys including a metal of the first series of transitions, e.g. Fe, Co, Vn, Cr, Mn and one of the metals of the heavy rare earths group, such as terbium or gadolinium or dysprosium.
The magnetooptic effect of these magnetic materials has to do with the interaction of a rectilinear polarized light with the magnetic state of the material. This interaction may occur by transmission of the light through the material, and then the magnetooptic effect is known as the Faraday effect. It may also occur by reflection onto the magnetooptical recording medium; in that case, the magnetooptic effect is known as the Kerr effect.
The interaction of a rectilinear polarized light with the magnetic state of the magnetic recording material of a magnetooptical memory causes the electrical field vector to rotate in the plane perpendicular to the direction of propagation of the light. The direction of this rotation is a function of this magnetic state (defined in particular by the direction of magnetization in the material in the vicinity where it receives the rectilinear polarized light). To read the data recorded on a magnetic medium for magnetooptical recording, it is accordingly sufficient to detect the direction of rotation of the electrical field vector.
The manner in which the data in a magnetooptical memory can be read is discussed in further detail in the aforementioned U.S. Pat. No. 4,510,544.
Magnetic recording media having properties such that the signal-to-noise ratio of the optoelectronic reading device associated with this medium is high are known. Such media are described, for example, in the article by G. A. N. Connel, R. Allen and M. Mansuripur of the Xerox Corporation Research Center in Palo Alto, California, entitled "Interference enhanced Kerr spectroscopy for very thin absorbing films--applications to amorphous terbium iron", published in the Journal of Magnetism of Magnetic Materials, No. 35, 1983, by the North Holland Publishing Company.
One such medium having magnetooptical properties is multilayered. It includes, in succession, a reflecting layer, typically metallic, disposed on a substrate; a first dielectric layer; a layer of magnetic material having magnetooptical properties, specifically an alloy preferably including iron or cobalt and a heavy rare earth, such as terbium or gadolinium; and a second dielectric layer. The mode of operation of a multilayered medium such as this is discussed in U.S. Pat. No. 4,666,789.
It will be recalled that the thickness of the layers of such a medium is on the order of several tens to several hundred Angstroms and may even attain one or two thousand Angstroms, and that the magnetic layer is anisotropic, with its axis of easy magnetization being perpendicular to its surface. In the aforementioned U.S. Pat. No. 4,666,789, it is indicated that such magnetic recording media have one major disadvantage, namely that their magnetic properties do not remain constant over time. In particular, depending on whether the initial content X of rare earth in the iron/rare earth alloy comprising the magnetooptical layer is less than or greater than a predetermined value for to each alloy, known as the compensation composition X.sub.comp, it is observed that:
if X&lt;X.sub.comp, the coercive field H.sub.c of the magnetic recording medium decreases with time, while
if X&gt;X.sub.comp, it is observed that the coercive field H.sub.c increases with time, and then decreases once the composition X has become less than X.sub.comp.
This phenomenon is a hindrance, because the coercive field of the magnetic recording medium must remain between certain limits, for example between 500 and 1000 Oerstads.
For the sake of simplification, this deterioration in the magnetic properties of the multilayered recording medium over time will hereinafter be called aging of the recording medium.
The aforementioned U.S. Pat. No. 4,666,789 describes and claims solutions for overcoming this disadvantage.
One of the solutions adopted comprises using a dielectric layer made of alumina and having a thickness that must be greater than 600 Angstroms, for protecting the medium. It has been possible to determine that over a period of time equal to 1000 hours, the magnetic properties of such a medium, covererd with an alumina dielectric layer, remain satisfactory at ambient temperature.
Nevertheless, it has been found that the resistance to aging of this medium deteriorates when temperatures higher than ambient temperature are used, and the extent of deterioration increases, the higher the temperature used. In other words, the higher the temperature at which the medium is used, the more the magnetic properties of the medium deteriorate.
This is an extreme hindrance when the thermo-magnetic writing mode is used.